1. Field of the Invention
The present invention relates to a light-controlled oscillator which shifts an oscillation frequency when light is applied.
2. Description of Related Art
A light-controlled oscillator is disclosed in JP-A-H11-298034. In this light-controlled oscillator, a transistor, whose characteristics change when light is applied, is used in a microwave circuit formed in a monolithic microwave integrated circuit (MMIC). By controlling ON/OFF of the light, this oscillator generates a high-frequency signal which is modulated by frequency shift keying (FSK) modulation.
In the FSK modulation, phase noise correlates with communication quality. Therefore, it is necessary to increase a width of frequency shift in the FSK modulation in order to reduce an influence of the phase noise.
In the above light-controlled oscillator, it is necessary to increase intensity of the light applied to the oscillator in order to ensure a large width of frequency shift. However, when the intensity of the light is increased, the phase noise increases or a frequency stability decreases because a probability of generation and disappearance of carrier in a semiconductor layer reacting to the light increases. Therefore, even when the intensity of the light is increased, communication quality cannot be improved sufficiently.